Die for producing a casting and method for making the die

ABSTRACT

The invention concerns a die for producing a casting, particularly a component for a motor or other engine, from a reactive molten non-ferrous metal charge using a casting mould and a method for manufacturing the die. The die consists of a casting mould which has a front layer area which comes into contact with the reactive non-ferrous molten metal charge and a top layer area mechanically stabilising the front layer area, the front layer area consisting of a minimum of one rare earth oxide and a minimum of one additional metal oxide, the minimum of one additional metal oxide being selected from the group of oxides of the following metals: titanium and nickel.

The invention concerns a die for producing a casting, particularly a component for a motor or other engine, from a reactive non-ferrous molten metal charge using a casting mould, and a method for manufacturing the die and the use of the die to produce a component.

BACKGROUND TO THE INVENTION

The use of casting moulds includes their employment for producing castings of the widest range of types. In the process, a molten charge of a casting material is introduced into a cavity of the casting mould. The casting material then solidifies, giving the casting an external shape which corresponds to that of the casting mould. The casting is subsequently removed from the casting mould. In particular, thin-walled casting moulds, known as ceramic shell moulds in the lost wax casting process, are frequently used.

In the lost wax casting process, materials based on intermetallic compounds are used to produce castings, particularly for high-temperature applications in motive engines. γ-titanium aluminides (γ-TiAl), nickel aluminides and nickel-based alloys are considered particularly suitable. Compared to traditional titanium materials, TiAl alloys are distinguished by better creep properties and lower susceptibility to oxidation.

DE 103 46 953 A1 discloses a die for producing castings and a process for manufacturing the die. The known casting mould is particularly suitable for handling TiAl alloys. The casting mould is manufactured using the slurry dipping and sand coating process. A wax blank in the shape of the casting to be made is prepared, repeatedly coated with a slurry, sand coated and then dried and cured. Finally, the wax blank is removed from the casting mould thus formed. A slurry consisting of water, yttrium oxide, magnesium oxide and calcium oxide is used to produce the known casting mould. In this way, a casting mould is obtained in which at least one surface coming into contact with the reactive non-ferrous molten charge consists of yttrium oxide, magnesium oxide and calcium oxide.

In connection with the use of different slurry compositions, it is also known that further additives matched to the binder system can be added to the slurries, as well as to the ceramic fillers, particularly liquefiers, wetting agents, condensers and antifoaming agents. Liquefiers improve the flow properties and affect the viscosity. Wetting agents improve the wettability of the wax blanks or coatings already present on dipping. The slurry reacts to the addition of even minimal volumes of both constituents, but does not display any pronounced sensitivity to slight “overdoses”. Viscosity which is too low can be readjusted by the addition of condensers. Even the lowest concentration of condensers is highly effective. They must be introduced in very small volumes during constant stirring, to prevent agglutination of the slurry. The antifoaming agent is primarily intended to prevent bubbles from forming on the surface of the slurry, which could cause defects in the production of the casting mould. Acids and bases can also be added. Complicated, time-consuming tests are usually necessary to create a slurry system suitable for the purposes of specific applications.

Casting moulds/ceramic shell moulds for handling intermetallic compounds, particularly those of reactive TiAl alloys, are known in which the surfaces of the casting mould/ceramic shell mould facing the front are made of AI₂O₃/Y₂O₃ (G. W. Dickhues: Feingusstechnologie für intermetallische γ-TiAI-Legierungen [Lost wax casting for intermetallic γ-TiAl alloys], Reihe 5 [Series 5]: Grund- und Werkstoffe [Primary matter and materials], no. 369, VDI-Verlag GmbH, 1994). The casting moulds known as ceramic shell moulds based on the Al₂O₃/Y₂O₃ or Al₂O₃ system are also made by dipping and sand coating within the scope of the known dipping process.

Dickhues represents an attempt to produce SiO₂-free ceramic shell moulds, as earlier lost wax casting methods used slurries which contained not only various ceramic filler systems but also primarily binders in the form of aqueous or alcoholic SiO₂ dispersions (silicic acid brine or ethyl silicate). The reason is that SiO₂ can be reduced by reactive molten charges, such as alloys based on TiAl or NiAl in ceramic shell moulds containing SiO₂. The consequence is considerable oxygen absorption by the molten charge, which may entail drastic losses of strength by the material. In addition, the interaction between the molten charge and the ceramic shell mould may entail pronounced erosion, which may cause the ceramic shell mould/casting mould to be destroyed, the molten charge to absorb ceramic particles and/or poor surface quality of the casting. Dickhues uses acrylic acid ester as a binder for the slurry for the front layers of the casting mould.

Dickhues not only conducted trials of the production of suitable casting moulds/ceramic shell moulds, but also of the production of crucibles which could be used for melting the material subsequently to be cast. Ceramic front layers of a crucible were produced by dipping into a Y₂O₃-titanium chelate slurry and coating with Y₂O₃ sand. However, it was not possible to make a reproducible crucible with a front layer on an yttrium oxide basis. Pronounced cracks consistently appeared when the casting moulds were fired, rendering the crucibles unusable.

Such cracking is presumably attributable to poor fusion of the particles of yttrium oxide to form a mechanically stable layer.

SUMMARY OF THE INVENTION

The purpose of the invention is to create a die for producing a casting from a reactive molten charge using a casting mould and a method for manufacturing the die which facilitates reliable, reproducible production of the casting mould.

According to the invention this problem is solved by a die for producing a casting in accordance with independent claim 1 and by a method for manufacturing a die in accordance with independent claim 12. Moreover, independent claim 26 refers to the use of a die to cast a casting.

According to the invention, a die is made for producing a casting; particularly a component for a motor or other engine, from a reactive non-ferrous molten metal charge using a casting mould; which die has a front layer area which comes into contact with the reactive non-ferrous molten metal charge and a back-up layer area mechanically stabilising the front layer area, the front layer area consisting of a minimum of one rare earth oxide and a minimum of one additional metal oxide, the minimum of one additional metal oxide being selected from the group of oxides of the following metals: titanium and nickel.

In accordance with a further aspect of the invention, a method is provided for manufacturing a die for the production of a casting, particularly a motor or other engine component, from a reactive non-ferrous molten metal charge, which method includes the following steps: providing of a wax blank with an external shape which reproduces the casting to be made; formation of a casting mould which has a front layer area which comes into contact with the reactive non-ferrous molten metal charge and a back-up layer area mechanically stabilising the front layer area, the wax blank being coated at least once with a slurry; drying and curing the casting mould; removal of the wax blank from the casting mould and firing of the casting mould; the front layer area of the casting mould being formed as a layer consisting of a minimum of one rare earth oxide and a minimum of one additional metal oxide, the minimum of one additional metal oxide being selected from the group of oxides of the following metals: titanium and nickel.

The particular advantage over the state of the art is that the sintering temperature for the front layer area is reduced and thus achieves improved fusion and ultimately improved physical properties.

A purposeful embodiment of the invention anticipates the minimum of one rare earth oxide being yttrium oxide.

In a preferred embodiment, as the at least one additional metal oxide a metal oxide is used which is an oxide from the same metal comprised in the reactive non-ferrous molten metal charge to be used in the casting mould. Such metal oxide may therefore be referred to as a metal oxide of the same species, namely the same metal. Thereby, a possible contamination of the non-ferrous molten metal charge by external or parasitic elements can be minimized or even completely avoided. Finally, such positive effect refers also to the casting produced using the die. The generation of undesired phases in the casting is avoided. In a further preferred embodiment, exactly one metal oxide of the same species is used in the front layer area. In case of casting a reactive non-ferrous molten metal charge based on a titanium alloy or a titanium aluminide, therefore, in an embodiment of the invention a titanium oxide is used in the front layer area of the casting mould. In a similar way, it is preferred to use a nickel oxide in case of a reactive non-ferrous molten metal charge based on a nickel alloy. There is the advantage of minimized oxygen penetration into the non-ferrous molten metal charge during production of the casting. In addition, the behaviour during the sinter process is farther improved.

A purposeful embodiment of the invention anticipates the yttrium oxide having purity in excess of 99.9%, avoiding problems which could occur due to the presence of impurities in the front layer area.

A purposeful further development of the invention may provide for the ratio by weight between the minimum of one rare earth oxide and the minimum of one metal oxide exceeding 10:1.

In one embodiment of the invention, the front layer area and/or the back-up layer area are purposefully produced by the slurry dipping and sand coating process. This method has the advantage over other methods for manufacturing casting moulds/ceramic shell moulds that even contours with the small dimensions of the wax blank casting reproducing the casting to be produced are coated reliably.

To minimise the disruptive effect of SiO₂ in the front layer area, an advantageous further development of the invention provides for the casting mould in which the front layer area which comes into contact with the reactive non-ferrous molten metal charge contains less than 0.1% SiO₂ by weight. This avoids unwanted reactions between the material of the front layer area and the molten charge.

A preferred embodiment of the invention anticipates that the casting mould in the front layer area and the back-up layer area is openly porous, rendering the casting form gas-permeable. A preferred embodiment of the invention provides for the casting mould in the back-up layer area having pores making up a volumetric proportion of between 20% and 40%.

A purposeful further development of the invention may provide for the casting mould having a surface roughness (Ra) better than 3.2 μm. Such high-quality surfaces are particularly advantageous to fluidic applications in motor and other engine components.

Such an advantageous embodiment provides for the casting mould having a strength in excess of 13 MPa. Such casting mould strengths are particularly suitable for centrifugal casting.

It is purposeful for the back-up layer area to be formed in various grain sizes, using aluminium oxide.

A purposeful further development of the invention anticipates the back-up area being produced using a minimum of one slurry containing an aqueous binder with an SiO₂ content. In this way, the stronger binding effect of SiO₂, which leads to increased strengths of the back-up layer and thus of the entire casting mould, can be used.

In a preferred further development of the invention, a further reduction in the sintering temperature is achieved by forming the front layer area by using a very fine-grained powder, particularly a powder consisting of nanoparticles, such as yttrium oxide nanopowder. It is known that the sintering temperature of nanopowders is up to several hundred K below the sintering temperature of microscale powders.

One embodiment of the invention provides using a mixture of the minimum of one rare earth oxide and oxides of the metals which principally occur in the molten charge, e.g. TiO₂ in the case of titanium alloys.

The die with the casting mould can be used to cast a casting by introducing a non-ferrous molten metal charge into the casting mould, cooling and solidifying the non-ferrous molten metal charge in the casting mould so that a casting is formed, and removing the casting from the casting mould.

The dependent claims for the method for manufacturing the die for producing a casting show advantages named in connection with the relevant properties in the dependent claims for the die for producing a casting.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

An example of the invention is explained below in greater detail using preferred specimen embodiments.

The production of a casting mould or ceramic shell moulds is based upon the slurry dipping and sand coating process, which is known as such in the field of lost wax casting and which includes the following process stages:

1) Dipping a wax model into a ceramic slurry

2) Sand-coating the dipped wax model with a ceramic powder

3) Drying the sand-coated wax model

4) Possible repetition of stages 1) to 3)

5) De-waxing the green form

6) Firing the green form

At least two different slurries are used: one to produce a front layer (FS) area facing a later casting and another to create a mechanically-stabilising back-up layer area (BS). The sand coating materials are adapted to the slurry.

The ceramic components of the slurry made from an aqueous solution for the front-layer area are Y₂O₃ powder with grain sizes between 1 μm and 50 μm and purity better than 99.9% and TiO₂ with a grain size of less than 10 μm. The aggregate proportion of ceramic substances is greater than 80% by weight. The ratio of the proportions (Y₂O₃/TiO₂) is greater than 10:1. The residual proportions of the slurry are water, a water-soluble SiO₂-free binder and discretionary further additives usual in lost wax casting such as liquefiers, wetting agents and/or antifoaming agents. Independent of the present embodiment, the slurry is a mixture of the different components.

A further slurry is used for the back-up layer area, with a solid content of approximately 60%, consisting of aluminium oxide powder mixed with powders with three different grain sizes. Other constituents are an aqueous binder containing SiO₂ on a basis of silicic acid, water, wetting agent and antifoaming agent.

A wax blank, the external shape of which corresponds to an article to be produced later with the casting mould, is dipped in the slurry for the front layer to form the front layer area. After the slurry has drained, the dipped wax blank is sand-coated with Y₂O₃ powders with purity better than 99.9% and a grain diameter from 50-250 μm. A low fine-grain proportion is advantageous. These stages in the process are repeated several times with interim drying of the layers until the desired front layer thickness has been obtained.

The wax blank bearing the formed front layer area is dipped in the additional slurry for the back-up layer area. It is subsequently dried and sand coated with a mixture of commercial

Al₂O₃ powders with three different grain sizes from 10-250 μm. These stages in the process are also repeated several times with interim drying of the layers until the desired back-up layer area thickness has been obtained.

The final drying of the green form takes place for about 48 hours at room temperature following the final immersion in the additional slurry for the back-up layer area. Superheated steam in a steam autoclave is used for subsequent de-waxing of the wax blank from the green form.

The de-waxed casting mould/ceramic shell mould is fired in air using a specified temperature gradient. The time curve is particularly characterized by a specified heating rate, at least one arrest point at a specific temperature below 800° C. and a maximum temperature below 1600° C. Cooling takes place slowly in a switched-off kiln. Cleaning of the ceramic shell mould follows, before it can be used for casting.

The characteristics of the invention disclosed in the above description and the claims may be significant to implementation of the invention in its various embodiments, either individually or in any combination. 

1. A die for producing a casting, particularly a component for a motor or other engine, from a reactive non-ferrous molten metal charge with a casting mould which has a front layer area which comes into contact with the reactive non-ferrous molten metal charge and a back-up layer area mechanically stabilising the front layer area, characterized in that the front layer area consists of a minimum of one rare earth oxide and a minimum of one additional metal oxide, the minimum of one additional metal oxide being selected from the group of oxides of the following metals: titanium and nickel.
 2. The die according to claim 1, characterized in that the minimum of one rare earth oxide is yttrium oxide.
 3. The die according to claim 2, characterized in that the yttrium oxide has purity in excess of 99.9%.
 4. The die according to claim 1, characterized in that the ratio between the minimum of one rare earth oxide and the minimum of one metal oxide is greater than 10:1 by weight.
 5. The die according to claim 1, characterized in that the front layer area and/or the back-up layer area are made by the slurry dipping and sand coating process.
 6. The die according to claim 1, characterized in that the casting mould in which the front layer area which comes into contact with the reactive non-ferrous molten metal charge contains less than 0.1% SiO₂ by weight.
 7. The die according to claim 1, characterized in that the casting mould in the front layer area and the back-up layer area is openly porous, rendering the casting mould gas-permeable.
 8. The die according to claim 7, characterized in that the casting mould in the front layer area and the back-up layer area has pores making up a volumetric proportion of between 20% and 40%.
 9. The die according to claim 1, characterized in that the casting mould in the front layer area has a surface roughness (Ra) better than 3.2 μm.
 10. The die according to claim 1, characterized in that the casting mould has a strength in excess of 13 MPa.
 11. The die according to claim 1, characterized in that the back-up layer area is formed in various grain sizes, using aluminium oxide.
 12. A method for manufacturing a die for the production of a casting, particularly a motor or other engine component, from a reactive non-ferrous molten metal charge, which method includes the following steps: Preparation of a wax blank with an external shape which reproduces the casting to be made; Formation of a casting mould which has a front layer area which comes into contact with the reactive non-ferrous molten metal charge and a back-up layer area mechanically stabilising the front layer area, the wax blank being coated at least once with a slurry; Drying and curing the casting mould; Removal of the wax blank from the casting mould and firing of the casting mould; and Firing of the casting mould; the front layer area of the casting mould consisting of a minimum of one rare earth oxide and a minimum of one additional metal oxide, the minimum of one additional metal oxide being selected from the group of oxides of the following metals: titanium and nickel.
 13. The method according to claim 12, characterized in that yttrium oxide is used as the minimum of one rare earth oxide.
 14. The method according to claim 13, characterized in that the yttrium oxide used has purity in excess of 99.9%.
 15. The method in accordance with claim 12, characterized in that the ratio of the minimum of one rare earth oxide to the minimum of one metal oxide is set greater than 10:1 by weight.
 16. The method in accordance with claim 12, characterized in that the front layer area and/or the back-up layer area are made by the slurry dipping and sand coating process when forming the casting mould, by the wax blank being dipped once or more into at least one slurry and subsequently discretionarily sand-coated.
 17. The method in accordance with claim 12, characterized in that at least one slurry is formed at feast partly with powder of the minimum of one rare earth oxide or of the minimum of one metal oxide, with a grain size measured in nanometres.
 18. The method in accordance with claim 12, characterized in that at least one slurry is water-based.
 19. The method in accordance with claim 12, characterized in that the casting mould in which the front layer area which comes into contact with the reactive non-ferrous molten metal charge contains less than 0.1% SiO₂ by weight.
 20. The method in accordance with claim 12, characterized in that the casting mould in the front layer area and the back-up layer area is openly porous, rendering the casting form gas permeable.
 21. The method according to claim 20, characterized in that the casting mould in the front layer area and the back-up layer area has pores making up a volumetric proportion of between 20% and 40%.
 22. The method in accordance with claim 12, characterized in that the casting mould in the front layer area has a surface roughness (Ra) better than 3.2 μm.
 23. The method in accordance with claim 12, characterized in that the casting mould is formed with a strength in excess of 13 MPa.
 24. The method in accordance with claim 12, characterized in that the back-up layer area is formed in various grain sizes, using aluminium oxide.
 25. The method in accordance with claim 12, characterized in that the minimum of one additional metal oxide is a metal oxide made from a metal comprised in the reactive non-ferrous molten metal charge.
 26. Use of a casting mould in accordance with claim 1 to produce a casting by introducing a non-ferrous molten metal charge into the casting mould, cooling and solidifying the non-ferrous molten metal charge in the casting mould so that a casting is formed and removing the casting from the casting mould.
 27. The use of a casting mould according to claim 26, where a nickel alloy, a titanium alloy or a material forming an intermetallic compound, particularly a TiAl or an NiAl alloy, is used as the non-ferrous molten metal charge. 